As a community, aquarists tend to become paranoid about 'hitch hikers', such as aiptasia (glass or triffid) anemones, and the entire clade of vermetid snails. As an online subculture, the community talks of these creatures as indestructible and existential threats, something more like fictional, movie xenomorphs, than what they are, real animals. Usually there exists a kernel of truth behind such fears, although aquarists folklore ignores the basic knowledge, that is relevant to their management - something more realist than a 'cure'. It seems strange today, but in the 1990s, any non-sedentaey polychaete could cause the majority of aquarists to panic. Of course the majority of them are harmless, and even helpful to the aquarist, because the common species are detrivores.
If the world of macroscopic plants has a counterpart in the 'reefer' imagination, it is surely that curious seaweed called bubble algae, or in other sorts of literature, sailor's eyeballs. We often call them all Valonia, and - with the caveat that I am not a botanist - I see no basis to doubt this. However the junior synonym Ventricaria is sometimes in use. These peculiar seaweeds - for they are unusual, green macroalgae - are all quite similar to one another, although some invest more energy in a larger 'bubble' - a spheroid to avoid body (or thallus), that is attached firmly to an underlying substrate - whilst others form a more creeping mat of smaller 'bubbles', because they may generate a stolon. It is the latter growth morphs that becomes invasive in our aquariums.
An curious and only half true bit of folklore about Valonia sp. is that they are the world's largest single celled organisms. Wether or not they are single celled is a matter of opinion, since they are miltinucleate, as are other green seaweeds of 'siphonous' type. Siphonous seaweeds are known to exist only among the green algae, with no counterparts among the brown or red algae, and to complicate things more, true multicellular algae have also evolved from non-siphonous, green algal ancestors. The 'giant cells' of siphonous algae contain numerous nuclei, each with its own 'cytoplasmic domain', whereas typical cells - of classically unicellular or multicellular organisms - possess but one cell nucleus. No cell wall separates the nuclei in the siphonous species, which is a basic difference from the truly multicellular plants.
Siphonous macroalgae thus exist outside of the familiar division of organisms into single or multiple cells, as we tend to take it for granted. For over one hundred years it has been pointed out, anyway, that the idea of a 'unicellular' organisms is flawed, on both semantic and empirical grounds. To begin with an organisms without cell walls is undivided, and therefore it cannot contain cells; also the 'cells' of what we call 'unicellular life' do not function entirely like the cells which act as the mere compartments of larger, living entities.
It is a frustrating description, but it is one that has stuck, and siphonous algae - and similarly multinucleate organisms, such as the famous slime molds -are clumsily expressed as 'giant cells'. If we view them as unicellular, then they surely are the biggest 'single celled organisms' in the world, but it is not the bubble algae would hold that title. Much larger siphonous macroalgae exist - such as Caulerpa sp and Halimeda sp. - that would wear that particular crown, sharing as they do the same siphonous nature as Valonia, and yet growing much, much larger.
Some people think that crabs which eat Valonia actively spread or, by releasing it's spores which are stored inside the globular thallus. This is nonsense because no spores are inside the plant, due to its structure making this impossible. This is not how the bubble algae multiply, although any intact 'cytoplasmic domain' can regenerate to form a complete Valonia organism. This is true of other siphonous algae such as Caulerpa, and neither makes bubble algae unique nor invincible. Without interference the bubbles multiply through division, which is also normal for such 'giant cells'. They also release new individuals via pores on their cell wall, and Valonia actually resembles, in its organization, the reproductive organs of some other siphonous algae
Incidentally the Mithrax group of crabs, including the emerald crab, M. sculptus, effectively consume Valonia; the problem is, their diets are broader than algae alone, and they are opportunistic foragers of sessile animals such as corals, and even slow, motile prey, such as snails. The popular focus on the emerald crab as a control agent of Valonia, is because few other herbivores find them palatable. In the wild they seem to be held in check by other, competing algae, and Valonia is commonest where these competitors are relatively absent. Some of the saccoglossan snails may be found on bubble algae, but most saccoglossan species will not consume it.
This is not to say that no other animals eat Valonia, but their extracts are known to be unappetising to herbivorous fishes. The fact they seem to profit where sea urchins graze on other algae, also points to their general unpalatability. It's curious that Siganus argenteus, a strictly vegetarian siganid, shows an actual preference for Valonia as food, whilst to another siganid, S. spinosus, the extracts are distasteful. Two species of the acanthurid genus Naso - N. lituratus and N. unicornis - are also verified to consume these macroalgae, whereas Acanthurus nigrofuscus and Zebrasoma flavescens do not It seems that the will to eat Valonia has evolved in fishes inhabiting those habitats, where they are relatively common as a potential food source.
Valonia is an adaptable enough genus, and can be found in harsh situations such as the intertidal zone, where it might be exposed to tropical air for hours of the day. Unsurprisingly, hardy algae of this kind easily survive transport on damp but emersed 'live rock' whilst it is transported by aeroplane - more fragile organisms are much less likely to survive the long process of shipment and 'curing'. They can also be found as deep as 80 meters, showing a preference for relatively shaded and flow deficient microhabitats, for example between stones. They are common on solid substrates for this reason, and are found growing in the wild, in association with certain species of Caulerpa. Bubble algae may grow, in the wild, as epiphytes on larger marine plants, including an association with the mangroves.
Valonia sp. ought not to be conflated with the 'grape'-forming morphs within the genus Caulerpa, nor the red grape or bubble algae of the genus Botryocladia, which belongs to the classically multicellular red seaweeds. Newcomers to the hobby are confused by the grape-like bubbles of these plants. Both these genera have a very different growth habit and appearance, when they are compared to the Valonia - although the latter, too, are marine plants, despite their noteworthy growth form. And when they are viewed as what they are, siphonous algae, the genus Valonia are not so unusual or perplexing as they might at first seem.
